Storage devices such as a magnetic medium based disk drive can store data on circular, concentric tracks on a disk surface. A read-write head retrieves and records data on a magnetic layer of a rotating disk as the head flies on a cushion of air over the disk surface. When retrieving data, magnetic field variations are converted into an analog electrical signal, the analog signal is amplified, converted to a digital signal, and interpreted. To guarantee the quality of the information saved on and read back from the disk, the read-write head should be accurately positioned at the center of the track during both writing and reading, and the speed or frequency of read and write should be accurately controlled with respect to the rotating disk. A closed-loop control system can respond to servo information embedded in dedicated portions of a track on the recording surface to accurately position the head and synchronize the timing of drive operations such as read and write operations.
The servo information defines the position of the data tracks and is written with great accuracy in order for a closed-loop control system to operate properly. Typically, servo information is written on a disk surface as a radially extending set of spokes or wedges, e.g., servo wedges. In some implementations, a portion of a servo wedge at a particular track location can include a synchronization field, index mark, a gray coded track number, and two or more fine-positioned offset bursts configured in an echelon across the track. Head positioning relative to a track center can be determined and corrected, if necessary, by reading and noting the respective amplitudes and timings of the offset bursts.